Over years of use, or just siting around, these old 6 cylinders engine accumulate a lot of sediment in the bottom of the water jackets in the block. If the different fan doesn't seem help, try flushing the block out. Keep in mind that some of it gets like concrete.

51 GMC 100 with 59 chevy 261 under the hood.
3 on the floor with 3.42 gear.